NOx and other harmful substances are contained in the exhaust gas of an internal combustion engine. It is known that a NOx catalyst for purifying NOx in the exhaust gas may be provided in the exhaust system of the internal combustion engine to reduce discharge of these harmful substances. In this technique, when a storage-reduction type NOx catalyst is provided, the purification performance decreases as the amount of stored NOx increases, and therefore, a reducer is supplied to the storage-reduction type NOx catalyst by performing a rich spike control to reduce and release the NOx stored in the catalyst, for example.
A process of supplying a reducer to achieve a rich atmosphere of the storage-reduction type NOx catalyst in addition to raising the bed temperature of the storage-reduction type NOx catalyst to a temperature that enables SOx to be released is sometimes performed, in order to counteract SOx poisoning in which SOx in the exhaust gas stored in the storage-reduction type NOx catalyst results in a decreased purification performance (hereinafter referred to as a “SOx regeneration treatment”). In the SOx regeneration treatment, the reducer is also used for raising the bed temperature of the storage-reduction type NOx catalyst.
In the case of performing the SOx regeneration treatment, it is necessary to supply the reducer to the storage-reduction type NOx catalyst as described above to achieve the rich atmosphere of the storage-reduction type NOx catalyst with a theoretical air-fuel ratio or less and to raise the temperature of the storage-reduction type NOx catalyst to a sufficiently high temperature for reduction and release of the SOx.
In this case, since both the air-fuel ratio of the exhaust gas introduced to the storage-reduction type NOx catalyst and the temperature of the storage-reduction type NOx catalyst need to be controlled optimally, a technique of alternately performing control of bringing the air-fuel ratio of the exhaust gas to the theoretical air-fuel ratio or a rich state and control of maintaining the temperature of the storage-reduction type NOx catalyst at a high temperature has been developed (for example, see Patent Document 1).
As an example of means of supplying the reducer to the storage-reduction type NOx catalyst, an appropriate combination is known of a technique of a secondary injection in a cylinder of an internal combustion engine and a technique in which an addition valve of the reducer is provided on the upstream of the storage-reduction type NOx catalyst in an exhaust passage and the reducer is added to exhaust gas passing through the exhaust passage (for example, see Patent Document 2 or 3).
One example of the control in the SOx regeneration treatment of the storage-reduction type NOx catalyst is control of alternately executing rich burn performed for burning in a cylinder of an internal combustion engine at a rich air-fuel ratio and lean burn performed for the burning in the cylinder at a lean air-fuel ratio. In this control, fuel supply unit that supplies the fuel to exhaust gas by a method different from an air-fuel ratio control during burning in the cylinder is provided. During a lean period in which the lean burn is executed, the fuel is supplied from the fuel supply unit to the exhaust gas to cause reaction in the storage-reduction type NOx catalyst, and the temperature thereof is maintained at a high temperature that enables the reduction and release of the SOx. During a rich period in which the rich burn is executed, a rich atmosphere of the storage-reduction type NOx catalyst is achieved to cause the reduction and release of the SOx.
Although the temperature of the storage-reduction type NOx catalyst is maintained at the high temperature that enables the reduction and release of the SOx during the lean period with the control described above, it has not been possible to perform the reduction and release of the SOx since the air-fuel ratio of the exhaust gas introduced to the catalyst is the lean air-fuel ratio. That is, the reduction and release of the SOx have not been performed during the lean period, and the reduction and release of the SOx have been performed only during the rich period. Thus, treatment time of the SOx regeneration treatment of the storage-reduction type NOx catalyst increases, and there has been a problem of deterioration of fuel efficiency or thermal degradation of the catalyst.    Patent Document 1: Japanese Patent Application Publication No. JP-A-2004-068700    Patent Document 2: Japanese Patent Application Publication No. JP-A-2003-120373    Patent Document 3: Japanese Patent Application Publication No. JP-A-2002-155724